High gain photo cells based on the work of Baumgartner, et al. in U.S. Pat. No. 5,769,384, “Low differential light level photoreceptors”, and improved by Knee, et al. in U.S. Pat. No. 6,104,020, “Electronic shutter for a low differential light level photo-receiver cell”, both assigned to Agilent Technologies, Inc., have been extensively used in the area of optical navigation. The photocells have an optically biased PNP bipolar transistor that serves as both the light to current converter as well as the first gain stage in the signal processing system. FIG. 1 illustrates a prior art photocell. The PNP transistor 40 is shown with the parasitic base-collector capacitor 38 formed by the reversed bias base-collector diode 34.
FIG. 2 (prior art) illustrates an emitter-base diode that must be forward biased to place the transistor in the forward active region. When light is sensed, there is a large DC component and a small AC component that establishes the operating point of the transistor. The AC component modulates the current in the emitter due to the high current gain at this operating point. The disadvantage of this circuit for pulsed light operation is that the signal current is consumed in reestablishing the forward bias on the base-emitter diode and is not available for integration on the signal capacitor 30 until the operating point of the PNP in the forward region is reestablished. The only current available to re-bias the transistor is the optically generated current corresponding to the input signal.
FIG. 3 present the integrated voltage on capacitor 30 versus time for DC and pulsed light conditions. Assuming an initial DC voltage on the capacitor 30 of Vref. For the DC case, there is a linear ramp down in time. For the pulsed case, there is a time Trb that is needed to rebias the transistor before the slope on the discharge rate is the same as the DC slope. During Trb, the current generated from the input signal is used to rebias the PNP transistor 40 and is not available for processing by the downstream circuitry. The amount of charge that is needed to rebias the PNP transistor 40 back to its operating point is to the first order constant. Therefore, the time Trb is inversely proportional to the signal current generated in the base. In an application, e.g. optical mouse, the light is provided by driving a light emitting diode (LED) to illuminate the surface. The need to preflash the LED to reestablish the bias point is a large power consuming function. In battery application, this is a major limitation on the time between battery replacements.